Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel canister is opened, allowing for intake manifold vacuum to be applied to the fuel canister. Simultaneously, a canister vent valve coupled between the fuel canister and atmosphere is opened, allowing for fresh air to enter the canister. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, fresh air flow within and through the canister is not uniform. Regions of adsorbent that see relatively less air flow will retain relatively more adsorbed hydrocarbons. Typically, 10-15% of the canister will retain some quantity of hydrocarbons following a purge operation, and this amount may increase as the canister ages. The residual hydrocarbons may desorb over a diurnal cycle, leading to an increase in bleed emissions. Strategies to limit these bleed emissions have included secondary canisters and heating elements, both of which increase manufacturing costs and require additional diagnostic testing.
The inventors herein have recognized the above issues and have developed systems and methods to at least partially address them. In one example, a method for purging a fuel vapor canister, comprising: responsive to a canister load decreasing below a first threshold, reversing a direction of air flow through the fuel vapor canister while maintaining purge air intake at a vent line inlet. By reversing the direction of air flow through the fuel vapor canister, canister regions that would normally retain fuel vapor during a purge event may realize increased desorption when the air flow direction is reversed. In this way, the efficiency of a purge event may be increased and bleed emissions may be reduced.
In another example, a method for purging a fuel vapor canister, comprising: coupling a first fuel vapor canister port to an engine intake; coupling a second fuel vapor canister port to a fresh air source; opening a canister purge valve coupled between the first fuel vapor canister port and the engine intake; and responsive to a fuel vapor canister load decreasing below a first threshold, coupling the second fuel vapor canister port to the engine intake and coupling the first fuel vapor canister port to the fresh air source. In this way, when purge flow has plateaued in a first direction, the flow of fresh air through the canister may be reversed, providing uniform purging throughout the canister.
In yet another example, a system for an engine, comprising: a fuel vapor canister comprising a fuel vapor canister buffer; a purge line coupling the fuel vapor canister to an engine intake via a canister purge valve; a vent line coupling the fuel vapor canister to a fresh air source; a canister vent valve coupled between the fuel vapor canister and the vent line, the canister vent valve operable between a first conformation and a second conformation; a reversing valve coupled between the fuel vapor canister buffer and the purge line, the reversing valve operable between a first conformation and a second conformation; and a controller comprising instructions stored in non-transitory memory, that when executed, cause the controller to: draw air through the fuel vapor canister with the canister vent valve in the first conformation and with the reversing valve in the first conformation; responsive to a fuel vapor canister load decreasing below a first threshold, draw air through the fuel vapor canister with the canister vent valve in the second conformation and with the reversing valve in the second conformation; and responsive to the fuel vapor canister load decreasing below a second threshold, lower than the first threshold, ceasing drawing air through the fuel vapor canister. In this way, bleed emissions may be reduced without requiring additional elements which may increase manufacturing costs, such as a canister heater or secondary bleed canister.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.